Hydroxyapatite in Whitlockite-Based Hypermineralized Hard Tissue of Elephant Shark (Callorhinchus milii) Tooth Plate

The elephant shark (Callorhinchus milii, Holocephali, Chondrichthyes) has unique dental organs, i.e., hypermineralized tooth plates. The mineral component of the hypermineralized tissue in the tooth plate is determined as whitlockite (WH: Ca9MgHPO4(PO4)6) regarding Chimaera phantasma, whereas hypermineralized tissues of bony vertebrates and elasmobranchs consist of hydroxyapatite (HAp: Ca5(PO4)3OH). Since holocephalans diverged from the same ancestor with elasmobranchs, it was speculated on phylogenetic grounds that holocephalans also have a mechanism to form HAp as a tissue component. However, at present, there is little crystallographic research on the hyper-mineralized tissue of holocephalans. To verify the above, we examined C. milii tooth plates using a high-performance microbeam X-ray diffraction technique. As a result, it was revealed that HAp formed in the early stage of the hypermineralized tissue formation; WH formed subsequently and the quantity increased rapidly to become the major phase. Nevertheless, low crystalline HAp was found to form throughout the tissue. The increment of WH synchronized with the increment of Mg in the tissue. Thus, it was suggested that although C. milii had a mechanism to form HAp, an excess of Mg2+ ions promoted transformation from HAp to WH. The presence of low crystalline HAp around WH may be related to hardening of the tissue.